Effect of Isomer Geometry on the Steady-State Absorption Spectra and Femtosecond Time-Resolved Dynamics of Carotenoids

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Abstract
Steady-state absorption and femtosecond time-resolved optical spectroscopic studies have been carried out on all-trans-β-carotene, 15,15‘-cis-β-carotene, all-trans-spheroidene, and 13,14-locked-cis-spheroidene. We examine in detail the effect of isomer geometry on the spectroscopic properties and photophysics of the low-lying S1 (21Ag-) and S2 (11Bu+) excited states of these molecules. The experiments on 13,14-locked-cis-spheroidene, a molecule incapable of undergoing cis-to-trans isomerization, provide a unique opportunity to examine the role of isomer geometry in controlling excited-state deactivation of carotenoids. The kinetic results have been obtained using both single wavelength transient absorption measurements and global fitting procedures. The overall scheme for the deactivation of these molecules after S0 → S2 photon absorption is decay of S2 to a vibrationally hot S1 state, followed by vibrational relaxation within S1, and finally, S1 → S0 internal conversion back to the ground state. Changes in isomer geometry are shown to lead to small but noticeable alterations in the spectroscopic and kinetic behavior of the molecules. The effects are interpreted in terms of minor alterations in excited-state energy and vibrational coupling upon isomerization that bring about changes in the spectroscopic and kinetic behavior of this biologically important class of pigments.